A disk drive used as a mass storage device in a digital data processing system typically includes multiple magnetic disks stacked on a single spindle. Data signals, which are stored on the magnetic disks as patterns of magnetic flux reversals, are recorded ("written") on the disks by means of read/write heads, that is, magnetic transducers. The data signals are written in concentric circular paths, or tracks, on one or both surfaces of all disks. Thereafter, read/write heads are used to retrieve ("read") the data signals recorded on the disks.
Each read/write head is associated with a surface, or portion of a surface, of a particular disk. The read/write heads, which are in close proximity to the various disk surfaces or tracks, are positioned laterally with respect to the disks by a head positioner. When the disk drive is to read or write data at a particular disk location, the head positioner positions the appropriate read/write head over the indicated disk location or track. The faster the head positioner positions the read/write head over the desired disk location, the faster the data can be transferred to or from the disk.
In a typical disk drive, a single positioner controls the positions of the various read/write heads associated with an entire stack of disks. The heads are attached to "rigid" support arms which in turn are attached to a rotor. The rotor rotates the support arms, and therefore the heads, about a spindle which is oriented parallel to the disk spindle. It thereby moves the heads laterally over the disk surface, moving each head to the same, (or approximately the same), relative position, that is, the same track, on the associated disk. This set of tracks is often referred to as a cylinder.
The invention pertains to rotational head positioners and, in particular, to "voice-coil driven"0 rotational positioners. To control the movement of the rotor, a voice-coil-driven rotational positioner utilizes the magnetic field associated with a current-carrying coil and a permanent magnet. The rotational direction of the rotor is governed by the direction of the current flow through the current-carrying coil. When the positioner is to move the heads to a particular disk location, current is passed through the coil in the appropriate direction, and a torque is developed in proportion to the current flowing in the coil and the magnetic flux density created by the permanent magnet in the coil region. As a result of the torque, the rotor is rotated, moving the heads.
Rapid acceleration and deceleration of the rotor, required for rapid positioning of the heads, cause reactive forces to develop in the rotor and the support arms. These reactive forces can interfere with the accurate positioning of the heads, particularly if they excite mechanical resonances in these structures.
One type of prior positioner essentially comprises a rotor with the support arms extending radially therefrom, and a rotational actuator, including the coil and the magnet, attached to one end of the rotor. The reactive torques of the rotor and arms induce torsion in the actuator shaft, and this adversely affects the rapid, accurate positioning of the read/write heads.
The torsion problem is largely eliminated in another type of rotary actuator, in which the actuator is integrated with the rotor to which the arms are attached. These actuators usually exert lateral forces on the rotational bearings that support the system. Thus the bearings must have high radial stiffness to counteract the lateral forces and prevent lateral movement of the rotor.
The high radial stiffness of the bearings, which are in contact with the rotor, is achieved by applying a high axial load to the bearings resulting in relatively large frictional torque which must be overcome before a stationary rotor may rotate. To overcome the frictional torque a torque is applied to the rotor. Thus there is a period when a torque is applied to the rotor and there is no corresponding rotor movement. This period is often referred to as a "dead band". Once the bearings/rotor frictional torque is overcome, the torque then being applied causes the rotor to rapidly rotate. Thus the applied torque and the rotor movement during and immediately after the dead band period are not proportional, often leading to read/write head positioning errors.